Add comprehensive locking around DB transactions

Transactional-fixture using tests with racing threads and inter-thread
synchronisation inside transaction blocks will now deadlock... but
without this, they would just crash.

In 5.0, the threads didn't share a connection at all, so it would've
worked... but with the main thread inside the fixture transaction, they
wouldn't've been able to see each other.

So: as far as I can tell, the set of operations this "breaks" never had
a compelling use case. Meanwhile, it provides an increased level of
coherency to the operational feel of transactional fixtures.

If this does cause anyone problems, they're probably best off disabling
transactional fixtures on the affected tests, and managing transactions
themselves.

activerecord/lib/active_record/connection_adapters/abstract/transaction.rb

@@ -149,6 +149,7 @@ def initialize(connection)
       end
 
       def begin_transaction(options = {})
+        @connection.lock.synchronize do
           run_commit_callbacks = !current_transaction.joinable?
           transaction =
             if @stack.empty?
@@ -161,8 +162,10 @@ def begin_transaction(options = {})
           @stack.push(transaction)
           transaction
         end
+      end
 
       def commit_transaction
+        @connection.lock.synchronize do
           transaction = @stack.last
 
           begin
@@ -174,14 +177,19 @@ def commit_transaction
           transaction.commit
           transaction.commit_records
         end
+      end
 
       def rollback_transaction(transaction = nil)
+        @connection.lock.synchronize do
           transaction ||= @stack.pop
           transaction.rollback
           transaction.rollback_records
         end
+      end
 
       def within_new_transaction(options = {})
+        @connection.lock.synchronize do
+          begin
             transaction = begin_transaction options
             yield
           rescue Exception => error
@@ -204,6 +212,8 @@ def within_new_transaction(options = {})
               end
             end
           end
+        end
+      end
 
       def open_transactions
         @stack.size

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -74,7 +74,7 @@ class AbstractAdapter
       SIMPLE_INT = /\A\d+\z/
 
       attr_accessor :visitor, :pool
-      attr_reader :schema_cache, :owner, :logger, :prepared_statements
+      attr_reader :schema_cache, :owner, :logger, :prepared_statements, :lock
       alias :in_use? :owner
 
       def self.type_cast_config_to_integer(config)

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -239,7 +239,9 @@ def truncate(table_name, name = nil)
 
       # Is this connection alive and ready for queries?
       def active?
+        @lock.synchronize do
           @connection.query "SELECT 1"
+        end
         true
       rescue PG::Error
         false
@@ -247,12 +249,15 @@ def active?
 
       # Close then reopen the connection.
       def reconnect!
+        @lock.synchronize do
           super
           @connection.reset
           configure_connection
         end
+      end
 
       def reset!
+        @lock.synchronize do
           clear_cache!
           reset_transaction
           unless @connection.transaction_status == ::PG::PQTRANS_IDLE
@@ -261,13 +266,16 @@ def reset!
           @connection.query "DISCARD ALL"
           configure_connection
         end
+      end
 
       # Disconnects from the database if already connected. Otherwise, this
       # method does nothing.
       def disconnect!
+        @lock.synchronize do
           super
           @connection.close rescue nil
         end
+      end
 
       def native_database_types #:nodoc:
         NATIVE_DATABASE_TYPES